Radko Kaspar

Grain boundary characterization and grain size measurement in an ultrafine-grained steel

Abstract Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass% C) were produced by large-strain warm compression and subsequent annealing treatment in a temperature range between 773 K and 1003 K. The samples were investigated by means of high-resolution electron back-scatter diffraction. The resulting microstructures showed very fine ferrite grains and homogeneously distributed cementite particles. The majority of the grain boundaries (55–70 %) were classified as high-angle ones (≥ 15° misorientation). When considering only these high-angle grain boundaries, the average grain size changed from 0.9 mm at a deformation temperature of 773 K to 2.2 mm at a deformation temperature of 1003 K. For the same range the average subgrain sizes increased from 0.6 mm to 1.5 mm. The basic result of this study is that the grain size characterization of polycrystalline microstructures with ultrafine grains requires the use of the high-resolution electron back-scatter diffraction method in conjunction with a careful a...

Measurement and Evaluation of the Anisothermal Softening of Austenite after Hot Deformation

A modified double-strain procedure is used to evaluate the anisothermal softening occurring in austenite during delay times after hot working. Both the rise in temperature due to deformation heating and the drop in temperature due to radiation during the rest time prior to reloading are taken into account. The technique requires prior determination of yield stresses in the unstrengthened test material over an adequate temperature range. By employing this method for anisothermal multi-pass hot deformation schedules, the state of austenite strengthening can be evaluated at any intermediate stage of working. Results obtained from plane-strain compression tests as a laboratory simulation of hot rolling under anisothermal conditions are presented for typical cases.

On the influence of heavy warm reduction on the microstructure and mechanical properties of a medium-carbon ferritic steel

Abstract The current study on a medium-carbon steel (0.36 % C) with deformation-induced spheroidized cementite produced by heavy warm deformation (HWD) was carried out in order to investigate an alternative microstructural route when compared to conventional quenching and tempering (QT) or soft annealing (SA) processes. After austenite deformation, the HWD samples were cooled and heavily deformed at temperatures below the γ–α transformation followed by a simulated coiling treatment. In particular, the study addresses the influence of adiabatic heating due to the heavy deformation on the microstructure and properties. The mechanical properties after the various treatments show that the strength–ductility relation after HWD and QT are superior to those after continuous cooling (CC) or soft annealing (SA). In the microstructures obtained from the CC and SA treatments we observed lamellar pearlite which deteriorated the mechanical properties. A similar effect was observed after the adiabatic HWD treatment, wh...

Einfluß der Austenitumformung auf die Umwandlung von Stählen

Die zielgerichtete Kombination der plastischen Umformung mit der Warmebehandlung wird als thermomechanische Behandlung (TMB) bezeichnet. Durch eine TMB last sich haufig die Festigkeit der Metalle bei gleichbleibenden Zahigkeitseigenschaften erhohen oder auch umgekehrt. Diese Art der Behandlung hat vor allem bei Stahlen grose Verbreitung gefunden, weil die allotrope γ - α-Umwandlung und die damit verbundene Anderung des Loslichkeitsvermogens fur andere Elemente ein besonders breites Spektrum von Moglichkeiten bietet. Die nach Anwendung einer TMB erzielten Verbesserungen der mechanischen Eigenschaften sind auf die Wechselbeziehung zwischen der verformungs- und legierungsbedingten Realstruktur des Austenits und den Umwandlungsvorgangen bei der anschliesenden Abkuhlung oder Warmebehandlung zuruckzufuhren. Die durch die Umformung eingebrachten Gitterfehler beeinflussen das Loslichkeitsverhalten, die Diffusion von Fremdatomen, die Selbstdiffusion und folglich sowohl die Keimbildung als auch das Wachstum der neuen Phase. Die Keimbildungsbedingungen werden durch das Auftreten lokaler Spannungsfelder um Versetzungsanhaufungen, Einschlusse und Ausscheidungen empfindlich beeinflust, indem die Koharenzbedingungen an den Umwandlungsgrenzflachen verandert werden /1/.

Control of a Servohydraulic Press for the Simulation of Hot Strip Rolling with the Plane Strain Compression Test

The rolling schedule of a multi-pass hot strip mill is simulated using the plane strain compression test with a computer controlled servohydraulic press. Actual rolling schedule data are converted by a user-written computer program into control signals for the testing machine. This includes the plastomechanical requirements for a desired average strain rate combined with a particular distribution of the strain rate curve. It also takes into account that at high strain rates or short interpass times the resulting ram displacement may differ greatly from the control signal. For this reason the machine characteristics have been calibrated. An electronic device combines the computer controlled ram movement with the motion of auxiliary tools. This ensures a reproducible and safe working sequence.